The present invention relates to a flexible pipe capable of being used for transporting fluids such as hydrocarbons for example.
Several types of flexible pipe are used. Some flexible pipes comprise, from the inside outwards, an internal sealing sheath made of plastic, elastomer or some other relatively flexible appropriate material; an unsealed flexible metal tube which has to withstand the loads developed by the pressure of the fluid flowing along the pipe; one or more plies of armours and at least one external sealing sheath made of a polymeric material. This type of flexible pipe is often termed a "smooth-bore" by specialists in this subject.
Other flexible pipes termed "rough-bore" comprise, from the inside outwards, an unsealed metal tube known as the carcass, consisting of a section wound into interlocked turns such as, for example, an interlocked metal strip or wire of an interlocking shape such as a wire in the shape of a T, U, S or Z, an internal sealing sheath made of a polymeric material, one or more plies of armours capable of withstanding the forces developed by the pressure of the fluid flowing through the pipe and the external forces to which the flexible pipe is subjected, and at least one external protective sheath of the polymeric type.
In the latter type of flexible pipe, the internal sealing sheath is directly extruded, continuously, over the carcass which, between the wound turns, has spaces or gaps.
To ensure good contact between the internal sealing sheath and the metal carcass, it is necessary for the inside diameter of the internal sealing sheath to be as close as possible and even equal to the outside diameter of the flexible metal carcass.
When manufacturing a flexible pipe of the "rough-bore" type, the internal sealing sheath which is extruded over the metal carcass shrinks onto the latter as it cools. Depending on the materials used to produce the internal sealing sheath, deformations known as "shrinkage cavities" appearing on the internal face of the said internal sealing sheath and particularly on each side of the gaps between the turns of the metal carcass can be observed after cooling. Such shrinkage cavities are due, it would seem, to the differential shrinkage of the material used for the internal sealing sheath, because of the variation in cooling gradient through the thickness of the internal sealing sheath, combined with the effect of the gaps between the turns of the metal carcass. What actually happens is that since the extruded plastic sealing sheath is in contact via its internal face with the metal carcass which is at ambient temperature, the cooling of the said internal face is very quick, and this causes surface irregularities or shrinkage cavities; this phenomenon is amplified at the gaps between the turns of the metal carcass, the differential shrinkage at these points leading to local variations in the thickness of the internal sealing sheath. When the sealing sheath is made of semicrystalline polymer, sensitive to the presence of surface defects leading to a deterioration in the sheath which may go so far as to rupture it, such as for example PVDF, this very often, in operation, leads to degradation of the sealing sheath (rupture) so that it then no longer fulfils its sealing function.
In order to remedy a drawback of this nature and to solve the problem which arises through the appearance of shrinkage cavities, the solution of depositing a thin sacrificial sublayer of an appropriate material such as PVDF between the metal carcass and the internal sealing layer was found and adopted. The internal sealing sheath is then extruded over the said sacrificial sublayer but making sure that there is no "welding" or intimate bonding between the sealing sheath and the "sacrificial" sublayer, so that cracks propagating from the internal face of the sublayer outwards are stopped at the interface between the sealing sheath and the sacrificial sublayer. This is what is described in WO 95/24578.
The major disadvantage of this solution is the slippage that is likely to occur between the internal sealing sheath and the sacrificial sublayer at the ends of the flexible pipe, and the additional cost in raw material and in transformation (manufacturing) caused by the presence of the said sacrificial sublayer.
It is also possible to produce a sacrificial sublayer in the form of a thin tape (maximum 2 mm thick) obtained from a homopolymer or a copolymer. Naturally, the extruded sheath, also known as the pressure sheath, and the sacrificial sublayer, in the form of a film or tape, exhibit deformation at the gaps and this allows the assembly consisting of the sheath and the sacrificial sublayer to catch on the interlocked metal strip of the internal carcass, the deformation not being sufficient to create shrinkage cavities on each side of each gap, because of the thermal conditions generated in the volume thus created.
Other solutions for eliminating the appearance of shrinkage cavities or for lessening their effects have been sought.
Among these last solutions, the purpose of which is to install an internal sealing sheath which, after cooling, has a smooth and cylindrical internal face, employ shaping which is either internal, with the main drawback that it creates longitudinal cracks on the internal face of the sealing sheath and folds of material on the external face, or external with the drawback of a complete absence of anchorage of the sealing sheath to the metal carcass.
In the technique for manufacturing flexible pipes of the "smooth-bore" type, which consists in producing separately the internal sealing sheath, using any appropriate means such as extrusion, and the metal carcass, it has been recommended that the sealing sheath or the metal carcass be heated once the two elements have been assembled, so as to keep the sealing sheath plastic or render it plastic in order to force it to creep into the gaps between the turns of the metal carcass. Such manufacturing methods are described in particular in FR-B-74 14 398 (COFLEXIP) and addition No. 71 16 880 (IFP).
However, the sole purpose of these methods is to cause permanent creep of the polymeric sealing sheath between the turns of the metal carcass after or at the same time as stresses are developed in the internal sealing sheath so as to bring about intimate contact, the stresses developed being due, for example, to a pressurizing of the said internal sealing sheath.
In an exemplary embodiment described in patent FR-B-74 14 398 and which relates to a flexible pipe comprising a peripheral sheath extruded over an assembly comprising, from the inside outwards, an internal sealing sheath, a pressure arch, two plies of armours and a metal lattice, it is recommended that the assembly be heated prior to extruding the peripheral sheath so as to keep at least the internal face of the said peripheral sheath in the plastic state or, more precisely, in the thermoplastic state so as, and this is the desired objective, to cause the internal face to creep into the meshes of the metal lattice to completely fill them and thus completely attach the peripheral sheath to the metal lattice. Under these conditions, it is essential that the assembly be heated strongly to temperatures of the order of several hundred degrees Celsius. Such techniques have yielded such poor results that they were very soon abandoned because the filling both of the spaces in the pressure arch and of the meshes of the metal lattice rigidified the pipe and therefore reduced the essential property of flexibility which it is imperative that it exhibit.
U.S. Pat. No. 3,311,133 describes a pipe comprising an internal metal carcass consisting of an interlocked S-shaped metal strip, in the gaps of which is inserted a compressible rod. The pursued objective is to control the spacing between the turns of the metal strip while at the same time ensuring that the said carcass has a certain flexibility. The rod recommended in this patent is made of a material which is dense although compressible and which has mechanical and plastic properties which are such that it cannot be used in the specific application of the present invention and which will be described later on. A disadvantage of the compressible rod of the prior art is that it is unable to take up the tensile load when being fitted between the turns of the internal metal carcass.
In French application 96 10 490 filed by the applicant company, it is recommended that the metal carcass be heated to a temperature of below 100.degree. C. upstream of the extrusion means so as to avoid sudden cooling of the internal face of the sealing sheath as it is extruded over the metal carcass. Work carried out on flexible pipes with preheated carcasses have demonstrated that the heat-induced creep of the internal sealing sheath was increased and that this sometimes caused the metal strip to lock. Such locking has the result of shifting the neutral axis in bending and therefore of increasing the deformation of the internal sealing sheath or pressure sheath on the outside of the bend. Now, when the complete flexible pipe is curved to the MBR (minimum bending radius) and if the strip is locked, it is easy to understand that for a deformation of the pressure sheath of the order of 6% on the outside of the bend, there is a deformation of practically about 10 to 12% on the inside of the bend, which is unacceptable. Furthermore, thermoplastics exhibit elongation at the threshold which reduces as the temperature decreases, which means that their capacity for deformation is also reduced at low temperatures. This phenomenon is of merely relative importance in the case of materials with a high elastic deformation, typically greater than 12% at the loading temperature, provided their capacity for deformation is maintained over time. By contrast, in the case of materials whose elastic deformation is limited, typically below 10 to 12% of the loading temperature, rupture may occur because this capacity for deformation is exceeded.
The use of thermoplastics whose capacity for deformation is high (greater than 10%) has not yielded good results because known thermoplastics are unable to withstand temperatures higher than 130 to 150.degree. C. (homopolymers or copolymers of PVDF) or exhibit other disadvantages such as poor creep strength (PFA). This is why, for an effluent temperature of about 130.degree. C., use is made of plasticized PVDF homopolymer. However, the plasticizer gradually disappears over time and this leads to an unplasticized thermoplastic which is unable to withstand the thermal and mechanical loadings. The loss of plasticizer is also a problem in that part of the pipe which is housed in the end fitting.
Furthermore, industrial implementation of a heated carcass and subsequent extrusion of the internal sealing sheath poses real problems. This is because the temperature-induced creep into the gaps varies according to the viscosity of the plastic used for the internal sealing sheath and the temperature to which the metal carcass is heated. As the physico-chemical properties of the said plastic may vary from one batch to another, it is practically impossible to have control over the temperature-induced creep under normal conditions of industrial implementation; complete temperature-induced creep has often been observed, that is to say complete filling of the gaps. It is easy to understand that complete temperature-induced creep may lead to locking of the metal carcass and therefore to reduced pipe flexibility. For example, when the filling of the gap by the internal sealing sheath is more than 90%, there is a risk that the flexible pipe will lock up at certain points.